1. Field of the Invention
The present invention relates to a macro lens having an exit pupil at a long distance for use with TV cameras with a color separation optical system, and, more particularly, to a macro lens which can form a sharp image of a subject located in a subject distance range from an infinite distance to the closest distance at which an image magnification is of approximately 1/3.
2. Description of Related Art
In order to produce fine quality close-up pictures, macro lenses typically have at least two groups of lens elements and are designed to move both the lens groups along the optical axis, changing an axial air spacing distance between the two lens groups. Such a macro lens is known from, for instance, Japanese Unexamined patent Publication No. 48-38,138.
In an attempt to decrease thrust movement of lenses or lens groups at very close focusing distances, some macro lenses of this kind are designed to cause a complicated relative movement of these lens groups along the optical axis so as to change an axial air spacing between each adjoining lens groups. Such a macro lens is known from, for instance, Japanese Unexamined patent Publication No. 58-186,714.
Both the macro lenses described in the above publications are designed for use with picture cameras and nothing is considered concerning a location of an exit pupil of the macro lens. Accordingly, the macro lenses are unavailable to color TV cameras having color separation optical systems which require a lens with an exit pupil located far away from an image plane of the lens. That is, as is well known to those skilled in the art, such a color separation optical system has a dichroic prism applied with an interference layer for separating light according to wavelengths. Since a macro lens having an exit pupil located at a relatively short distance from an image plane causes a principal ray to enter into the interference layer of the dichroic prism at greatly different incident angles depending upon image heights, color separation takes place differently according to lateral distances of principal rays from the optical axis due to a difference in optical path of the interference layer. If a principal ray is in parallel with the optical axis, in other words, if an exit pupil is located at infinity, the principal ray enters into the interference layer at a same incident angle independently from lateral distances of a principal ray from the optical axis, and color separation occurs with same characteristics. Although the allowable range of incident angle is slightly different according to dichroic prisms used in TV camera optical systems, it can be generally said that an incident angle of a principal ray less than one (1) degree is allowable.
An incident angle Ad of a principal ray is generally defined for an image having a height h by the following equation: EQU Ad=tan.sup.-1 h/f
where f is the focal length of a lens.
If a TV camera has a 2/3" image plane whose diagonal size is 11.0 mm. and whose aspect ratio is 3:4, a lateral height of an image is 3.30 mm. Accordingly, the focal length f is 189.0 mm. for an incident angle (Ad) of a principal ray of one (1) degree. Consequently, in order for a principal ray to enter the dichroic prism at an incident angle within one (1) degree, the focal length f must be longer than 189.0 mm. On the other hand, a thrust movement of lens X from an infinity focusing distance is defined for a magnification M by the following equation: EQU X=M.times.f
Accordingly, if the magnification M is 1/3, the thrust movement of lens X is 63.0 mm., which makes the overall length of the lens large.